The present invention relates to a Code Division Multiple Access (CDMA) mobile communication system having a feature in transmitting a signaling message and more particularly a CDMA mobile communication system having a feature in transmitting a signaling message which is advantageous to the reception of an Extended Handoff Direction Message on executing a soft handoff operation.
In a CDMA mobile communication system, an identical frequency zone is shared by a plurality of communication users. This may cause deteriorated communication quality because signals sent from other communicators may act as interference signals to the communication.
Particularly, when communications are simultaneously carried out using a mobile station located in the vicinity of a base station and another mobile station located remotely to a base station, signals from a near mobile station are received by the base station at high electric power level, while signals from a far mobile station are received at low power level. This produces substantially deteriorated communication quality to the remotely located mobile station, which is caused by the signal interference by the near mobile station. This problem is known as a xe2x80x9cnear-far problemxe2x80x9d.
In order to cope with above-mentioned problem, there has been studied to introduce a technique for controlling transmission power. The transmission power control method controls transmission power to maintain Eb/No at a constant level irrespective of the location of a mobile station. Here, Eb/No denotes a ratio of received energy (Eb) to received noise energy (No) respectively measured at a receiving station.
To realize the method, a power control bit has been introduced to insert in transmission frames to perform a xe2x80x9cclosed loop adjustment controlxe2x80x9d, by which, at a receiving station, transmission power is adjusted based on the received power control bit.
Specifically, an average frame error rate (FER) is measured at a base station to compare with a target FER. When a difference exists between the target FER and a measured average FER, a target Eb/No is updated so that an average FER reaches nearer to the target FER.
At the base station, Eb/No is measured to compare with a target Eb/No. If a measured Eb/No is greater than the target Eb/No, a power control bit is inserted in information signals of forward (downlink) frames to direct the mobile station to lessen the transmission power therefrom. On the other hand, if a measured Eb/No is smaller than the target Eb/No, a power control bit is inserted to direct the mobile station to increase the transmission power.
At the mobile station, the power control bit is extracted from information signals in received frames. Transmission power is adjusted according to the power control bit.
Similarly, at the mobile station, an average FER (frame error rate) is measured to compare with a target FER. When a difference exists between the target FER and the measured average FER, a target Eb/No is updated so that a measured average FER reaches nearer to the target FER. Also, Eb/No is measured in transmission signals from the base station to compare with a target Eb/No. If a measured Eb/No is greater than the target Eb/No, a power control bit is inserted in information signals of reverse (uplink) frames to direct the base station to lessen the transmission power output therefrom. On the other hand, if a measured Eb/No is smaller than the target Eb/No, a power control bit is inserted to direct the base station to increase the transmission power.
At the base station, the power control bit is extracted from information signals in received frames. Transmission power is adjusted according to the power control bit.
Accordingly, a closed loop adjustment control is carried out to control transmission power in cooperation with the mobile station and the base station.
On the other hand, call control sequences (call originating, terminating, handoff, and so forth) in a CDMA system are carried out independently to the transmission power control mentioned above. In the conventional technology, signaling messages are transmitted in call control sequences without any consideration on the condition of received radio wave.
Therefore, there are cases that signaling messages are transmitted even when a radio condition is deteriorated at the receiving side. In such cases, the transmitted signaling messages may not be received because of an occurrence of frame error.
To avoid risks of reception failure in conventional CDMA mobile communication system, such a measure has been introduced as a retransmission of a signaling message when an acknowledgement message cannot be received within a predetermined period after the relevant signaling message is transmitted.
However, the above-mentioned measure may cause another problem that the retransmitted signaling message newly becomes an interruption source against communications of other mobile stations.
It is accordingly an object of the present invention to provide a mobile communication system using CDMA (Code Division Multiple Access) scheme having a feature in signaling message transmission control which ensures a predetermined processing by avoiding the transmission of a signaling message when the radio receiving condition at the receiving side is deteriorated.
It is another object of the present invention to provide a mobile communication system which controls the transmission of a signaling message at the start of soft handoff procedure by which communication is carried out particularly among a plurality of base stations.
In order to achieve the aforementioned object, the transmission timing of a signaling message in the present invention is controlled considering the radio receiving condition at the reception side.
Preferably, in the present invention, a mobile communication system based on CDMA scheme comprises a plurality of mobile stations, a plurality of base stations, and higher rank equipment (i.e. a mobile switching center and a base station controller) for controlling and managing the plurality of base stations. In the base stations, a means is provided for transmitting a common traffic channel to which each plurality of mobile stations can refer.
In each of the higher rank equipment mentioned above, a table is provided for managing the busy-idle state of each traffic channel in each plurality of base stations. A means is also provided for transmitting information in the above-mentioned table to the plurality of mobile stations through the common traffic channel of the base station.
It is a feature of the present invention to provide the mobile stations with a means for reporting, during soft handoff process to communicate simultaneously with the plurality of base stations, to the base station currently communicating the received power of a pilot channel from the base station currently in communication, as well as the received power of a pilot channel from a target base station of soft handoff, and a short code of an idle traffic channel randomly selected with a random number or the like using busy-idle information on traffic channels of the target base station of soft handoff based on the table information received on the common traffic channel of the target base station.
Further, preferably, the above-mentioned common traffic channel is transmitted using a common short code and a common long code.
Still further, the above-mentioned higher rank equipment provides a table for managing the busy-idle state on traffic channels of base stations surrounding a base station currently communicating with a mobile station, the content of the table being transmitted from the base station to the mobile station through a traffic channel each time the table is updated.
At the time of soft handoff when simultaneously communicating with a plurality of base stations, the mobile station obtains report information on the received power of a pilot channel from the base station currently communicating, as well as the received power of a pilot channel from a target base station of soft handoff, and a short code of an idle traffic channel randomly selected using a random number or the like from idle traffic channels of said target base station extracted from traffic channel busy-idle information related to said surrounding base stations received from said higher rank equipment.
Still further, preferably, in the above-mentioned procedure the higher rank equipment provides a means for receiving the report information from the mobile station through the base station to examine using the table whether the short code specified in said received report information is idle. When the short code is determined to be idle, the higher rank equipment directs the target base station of soft handoff to acquire a reverse traffic channel of the mobile station to start to transmit a traffic data of the mobile station through the traffic channel specified by the short code. A means is also provided in the higher rank equipment that, with setting busy, state on the short code in the table, and transmitting the table information to the mobile station through the traffic channel of the base station currently communicating and through the common traffic channel of the target base station of soft handoff.
Still further, preferably in a mobile communication system adopting CDMA scheme which includes a plurality of mobile stations, a plurality of base stations and higher rank equipment for controlling and managing the base stations, a power control bit is transmitted and received between the mobile station and the base station, to control to inhibit the transmission of signaling messages to the mate stations when the power control bit indicates deteriorated receiving condition.
Also, preferably the aforementioned signaling message is an Extended Handoff Direction Message transmitted through a forward traffic channel from a target base station of soft handoff to a mobile station at the start of soft handoff operation.
Still further, preferably in a CDMA mobile communication system which includes a mobile station, a first base station and a second base station each having mutually neighboring coverage areas, and higher rank equipment for managing the first base station and the second base station, the higher rank equipment in the system provides a table for managing the busy-idle state of codes allocated to a forward traffic channel of the second base station, and at least either the first or the second base station provides a code information transmission means for transmitting to the mobile station the busy-idle information obtained from the higher rank equipment. The mobile station in the system provides; an information reception means for receiving the busy-idle code information; a selection means to select an idle code by use of information content received by the information reception means; a report means to report a signal to the higher rank equipment to specify the selected code through the base station at the time of handoff from the first base station to the second base station; and a reception means for performing a receiving operation on a traffic channel from the second base station to the mobile station itself by use of the selected code. In addition, the higher rank equipment provides a control means for controlling the second base station to start to transmit a traffic channel to the mobile station by use of the specified code when the report is received.